## test if the file is a regular file 
if [ -f 'file_test.sh' ]; then
    echo 'it is a regular file.'
else 
    echo 'it is not a regular file.'
fi

# the key is option '-t': there're many other options can
# be replaced:

# -d: test if given path is directory
# -h: test if given path is symbolic link
# -b: test if given path is block device
# -c: test if given path is character device
# -p: test if given path is named pipe
# -S: test if given path is socket
# -nt: if file1 is newer than file2


